Why are incompatible return types with wildcards compiled in common and non-common override methods?

Question

What is the reason for this strange behavior? List<? super Number>"narrower" ("more specialized") than List<?>, so the code in example 1 below will not work! When I have a List<?>return type in an overridden method in a base class (interface), it will always have List<? super Number>the return type of an override method in the Derived class (interface) - it compiles fine in examples like 1-3 and I understand why.

1. The Superinterface (I1) method is general, the subinterface (I2) method is not general. The covariant return in the subinterface (I2) compiles fine:

interface I1 {
    public <T> List<? super Number> f() throws IOException;
}

interface I2 extends I1 {
    public List<?> f() throws EOFException; // compiles! covariant return is OK! 
}

2. Both interface methods are common - compile an error in I2 . The Superinterface (I1) method is general, the subsequence method (I2) is also general. Returning covariance in subinterface (I2) gives a compilation error. "Return type is incompatible with I1.f ()":

interface I1 {
    public <T> List<? super Number> f() throws IOException;
}

interface I2 extends I1 {
    public <T> List<?> f() throws EOFException; // covariant return fails 
}

3. Both interface methods are not common - they compile an error in I2 . The Superinterface method (I1) is not general, the subsequence method (I2) is also not general. The covariant return in the subinterface (I2) gives the same compilation error as in 2 - "the return type is incompatible with I1.f ()":

interface I1 { 
    public List<? super Number> f() throws IOException;
} 

interface I2 extends I1 { 
    public List<?> f() throws EOFException; // covariant return fails 
}